This invention relates to an apparatus for a centrifugal dryer for separating and drying pellets according to the preamble of patent claim 1.
Centrifugal dryers are used for example for drying plastic pellets. Easy cleanability of the process spaces is important because when the pellet color or grade is changed one must prevent pellets or fragments of the preceding batch from mixing with the new one. Long service life of the apparatus parts, low abrasion of the pellets to be dried and low sound emissions are also important.
Known centrifugal dryers according to German laid-open prints DE 195 37 41 A1, DE 43 30 078 A1 and DE 28 19 443 A1 and according to U.S. Pat. No. 5,611,150 consist of a housing in which a hollow body formed of screens coaxially encloses a pivotally mounted, vertically disposed rotor equipped with conveyor blades. The screens either form a closed cylinder or are segmented. They extend from the bottom up to the pellet discharge zone located at the upper end, being interrupted only by fastening elements and possibly a feed chute.
The suspension of pellets and liquid is supplied continuously to the lower area of the centrifugal dryers, into the space between the rotor and the hollow body formed of screens. It is caught by the corotating conveyor blades of the rotor which transfer both an upward directed transport pulse and centrifugal forces to the suspension. The pellets are thereby both transported upward and thrown back and forth between the screens and conveyor blades, while the liquid separated from the pellets passes through the screens and is removed through the discharge port of the housing receiving the centrifugal dryer.
Furthermore, air is sucked from above into the process space, part of the air exiting with the dried pellets and the other part being blown or sucked off from a lower-lying area of the centrifugal dryers in countercurrent to the pellets. This air countercurrent prevents pellets from being remoistened with liquid droplets formed upon impingement of the pellets.
Such centrifugal dryers have proved successful in principle. A disadvantage is the contamination of the screens with fragments of the pellets so that said screens must be dismantled, cleaned or replaced before every change of color or grade. Furthermore, screens wear out relatively heavily, are easily damaged by frequent change, and produce abrasion on the pellets to be dried. They are therefore unsuitable for drying strongly abrading pellets such as plastic pellets filled with glass fibers. Also, the sound emitted by impingement of the pellets on the screens is considerable. Although the outside housings of such centrifugal dryers can be insulated against excessive sound emissions, this insulation is elaborate because the accessibility of the screens requires closable openings such as flap doors, etc., on the housings.
The invention poses the problem of providing a centrifugal dryer which reduces abrasion of the pellets.
For solving this problem a substantial part, preferably at least in the upper area of the hollow body, is formed as an unperforated jacket. The liquid separated from the pellets flows downward in a spiral shape on the inside wall of the unperforated jacket under the action of the centrifugal and gravitational acceleration. The unperforated jacket with its smooth inside surfaces causes little abrasion.
Depending on the wall thickness the unperforated jacket causes little sound so that the hollow body can simultaneously serve as an outside housing where it is formed as an unperforated jacket.
Preferably at most the lower area of the hollow body is equipped with screen segments, or the hollow body is formed completely as an unperforated jacket with a screen in the lower area of the hollow body.
The main advantage of the invention is that only small screen surfaces, if any, need be cleaned or replaced before a change of color or grade. Pellet residues adhering to smooth walls are easily rinsed off with the aid of washing nozzles or by flooding the total process space, whereas they become wedged in screen holes. In addition, the at least partial omission of an outside housing enclosing the process space reduces costs.